Experimental measurement of velocity correlations for two microparticles with ion wakes

Summary form only given. We study velocity distributions and correlations in a dusty plasma that contains only two polymer microparticles. We used 4.8 μm polymer microparticles, which became charged negatively when dropped into an argon rf glow-discharge plasma operated at 13.56 MHz and 13 mTorr. Due to their large charge, the microparticles were electrically levitated in the sheath above the horizontal electrode. Natural electric potentials in the glow discharge provided confinement for the microparticles in all directions, so that oscillatory modes dominated their random motion. Extending our earlier experiment¹, we shaped the levitating sheath two ways, so that the two microparticles were aligned either vertically or horizontally. This alignment direction is significant because the ion flow was in the vertical direction, and ion flow causes a wake potential to form downstream of a microparticle. This wake potential modifies the interparticle interaction and can lead to an instability that adds kinetic energy to the microparticle motion². By using only two microparticles, we are better able to distinguish the effects of ion wakes. We also designed the experiment to have symmetries for the microparticle confinement that allow distinguishing the effects of confinement from those of the wake potential.Using high-speed video microscopy as the diagnostic, we tracked the motion of the microparticles. From the records of the microparticle velocities, we calculated the distribution function f l(v) and the two-particle velocity distribution f 2. Our most significant results center on the correlation function g_ 2, which we calculated from the experimental velocity measurements as g_ 2 = f_ 2 - f_ 1 f_ 1. To characterize the oscillatory modes, we performed frequency filtering on the velocity data before computing g_2. The results reveal that when the microparticles are aligned parallel to the ion flow, the microparticle motion is more highly c- rrelated, and the oscillatory modes lack the shear motion that is present when the micro particles are aligned the other way, i.e., perpendicular to the ion flow. We can attribute these differences in the micro particle motion to the ion wake.